1. Field of the Invention
The present invention relates to a transfer sheet for use in the preparation of a three-dimensional object with a pattern transferred on the surface thereof.
2. Background Art
In transferring a pattern on the surface of a three-dimensional object, such as an injection molded article of a resin or a wooden shaped object, a vinyl chloride resin film capable of being formed into a three-dimensional shape has hitherto been extensively used as a substrate for a transfer sheet. Transfer sheets of this type are disclosed in, for example, (1) Japanese Patent Publication No. 69759/1994, (2) Japanese Patent Publication No. 29518/1995, and (3) Japanese Patent Publication No. 100398/1995. The transfer sheet (1) comprises a pattern layer, formed of a releasable cellulosic ink or the like, directly provided on a substrate. The transfer sheets (2) and (3) have the same construction as the transfer sheet (1), except that the pattern layer directly provided on the substrate in the case of the transfer sheet (2) and the release layer directly provided on the substrate in the case of the transfer sheet (3) are formed of one member selected from polyvinylbutyral, a cellulose acetate propionate resin and a chlorinated polypropylene resin.
In the transfer sheet (1), however, printing for directly providing the pattern layer on the substrate causes a solvent to attack the vinyl chloride resin film as the substrate, posing a problem of unstable releasability of the substrate. In the transfer sheet (2), a plasticizer contained in the vinyl chloride resin film as the substrate bleeds out, here again posing a problem of unstable releasability of the substrate. In the transfer sheet (3), resins usable in the release layer are disadvantageously limited to specific resins, that is, polyvinyl butyral, a cellulose acetate propionate resin, and a chlorinated polypropylene resin.
Thus, in decorating the surface of a three-dimensional object, the vinyl chloride resin film has good formability but is not always satisfactory as the substrate of the transfer sheet.
In order to overcome this problem, for example, (4) Japanese Patent Publication No. 110550/1995 discloses transfer sheets using as the substrate, besides the vinyl chloride film, other plastic films, such as polystyrene resin, acrylonitrile resin, and ABS resin films. In the transfer sheet (4), however, an anchor layer should be provided on the substrate in order to improve the adhesion of the plastic film to other layer(s). Therefore, the construction of the transfer sheet is inevitably such that a release layer, a pattern layer and the like are provided on the anchor layer. The presence of the anchor layer and the release layer narrows a suitable temperature range for thermoforming of the transfer sheet.
Accordingly, the present inventor has made tests and studies on a transfer sheet using a polyethylene film or a polypropylene film as the substrate film with a view to developing a transfer sheet not using the vinyl chloride resin film substrate and not requiring the provision of any auxiliary layer such as an anchor layer, the substrate film having suitable releasability and formability to a shape conforming to the shape of the object. The transfer sheet using the polyethylene or polypropylene film is effective also as an alternative to the transfer sheet using the vinyl chloride resin film which evolves hydrochloric acid gas upon incineration. The transfer sheet using the polyethylene or polypropylene substrate film raises problems including that the polyethylene or polypropylene film has high crystallinity and rapidly softens at the melting point, narrowing the range of acceptable forming conditions, and, in addition, when stretched by heat at the time of forming, creates partial whitening due to necking, causing the elongation of the whitened portion to be rendered larger than that of the other portion, that is, the elongation to become uneven.